Preterm labor, or labor before 37 weeks gestation, has been reported in approximately 12.8 percent of all births but accounts for more than 85 percent of all perinatal complications and death. Rush et al., BMJ 2:965-8 (1976) and Villar et al., Res. Clin. Forums 16:9-33 (1994), which are both incorporated herein by reference. An inverse relationship between cervical length in the formix vaginae and the risk of preterm labor has also been observed. Andersen et al., Am. J. Obstet. Gynecol. 163:859 (1990); tarns et al., N. Eng. J. M. 334:567-72 (1996) and Heath et al., and Ultrasound Obstet. Gynecol. 12:312-7 (1998), which all are incorporated herein by reference. Accordingly, many physicians find it useful to examine the cervix in the formix vaginae as part of normal prenatal care in order to assess risk of preterm labor.
It has long been known that the cervix normally undergoes a series of physical and biochemical changes during the course of pregnancy, which enhance the ease and safety of the birthing process for the mother and baby. For example, in the early stages of labor the tissues of the cervical canal soften and become more pliable, the cervix shortens (effaces), and the diameter of the proximal end of the cervical canal begins to increase at the internal os. As labor progresses, growth of the cervical diameter propagates to the distal end of the cervical canal, toward the external os. In the final stages of labor, the external os dilates allowing for the unobstructed passage of the fetus.
In addition to the physical and biochemical changes associated with normal labor, genetic or environmental factors, such as medical illness or infection, stress, malnutrition, chronic deprivation and certain chemicals or drugs can cause changes in the cervix. For example, it is well known that the in utero exposure of some women to diethylstilbestrol (DES) results in cervical abnormalities and in some cases gross anatomical changes, which leads to an incompetent cervix where the cervix matures, softens and painlessly dilates without apparent uterine contractions. An incompetent cervix can also occur where there is a history of cervical injury, as in a previous traumatic delivery, or as a result of induced abortion if the cervix is forcibly dilated to large diameters. Details of the incompetent cervix are discussed in Sonek, et al., Preterm Birth, Causes, Prevention and Management, Second Edition, McGraw-Hill, Inc., (1993), Chapter 5, which is incorporated by reference herein.
Cervical incompetence is a well recognized clinical problem. Several investigators have reported evidence of increased internal cervical os diameter as being consistent with cervical incompetence (see Brook et al., J. Obstet. Gynecol. 88:640 (1981); Michaels et al., Am. J. Obstet. Gynecol. 154:537 (1986); Sarti et al., Radiology 130:417 (1979); and Vaalamo et al., Acta Obstet. Gynecol. Scan 62:19 (1983), all of which are incorporated by reference herein). Internal os diameters ranging between 15 mm to 23 mm have been observed in connection with an incompetent cervix. Accordingly, a critical assessment in the diagnosis of an incompetent cervix involves measurement of the internal cervical os diameter.
There are also devices and methods to measure the diameter of the external cervical os. For example, cervical diameter can be manually estimated by a practitioner's use of his or her digits. Although an individual practitioner can achieve acceptable repeatability using this method, there is a significant variation between practitioners due to the subjective nature of the procedure. To address these concerns, various monitoring and measuring devices and methods have been developed. For example, an instrument for measuring dilation of the cervix uteri is described in U.S. Pat. No. 5,658,295. However, this device is somewhat large, leading to a risk of injury to the fundus of the vagina or cervical os. Additionally, it is not disposable and requires repeated sterilization. Another device for measuring cervical diameter is described, for example, in U.S. Pat. No. 6,039,701. In one version, the device described therein has a loop element which is secured to the cervix. The loop expands or contracts with the cervix and a gauge is coupled to the loop for measuring changes in the loop dimension. Such changes can then be detected by electronic means. Accordingly, this device is rather complex and expensive to manufacture.
Even if a woman is found to have an apparently normal internal cervical os diameter, there may nonetheless be a risk for preterm labor and delivery. Currently, risk assessment for preterm delivery remains difficult, particularly among women with no history of preterm birth. However, the findings that preterm delivery is more common among women with premature cervical shortening or effacement suggest that a measuring the length of the cervix would be predictive for preterm labor.
Currently, a physician has at least two options to measure the length of the cervix in the formix vaginae. One such method involves serial digital examination of the cervix by estimating the length from the external cervical os to the cervical-uterine junction, as palpated through the vaginal formix. Although this is useful for general qualitative analysis, it does not afford an easy nor accurate measurement of the length of the cervix from the external cervical os to the cervical-uterine junction (also described herein as the length of the cervix extending into the vagina) and, therefore, does not provide an accurate assessment of the risk of preterm labor. Despite the use of gloves, digital vaginal exams always carry with them the risk of transmitting infectious agents, especially to the fetal membranes, the lining and/or muscle of the uterus, or to the fetus itself.
Another method involves real-time sonographic evaluation of the cervix. This method provides relatively quick and accurate cervical dimensions. However, it requires expensive equipment, highly skilled operators, as well as skill in interpretation of results, which are all subject to human error. Additionally, there is a risk that the probe that must be inserted into the vagina as part of the procedure may cause injury if not inserted with care. Also, due to the expense of the procedure many women, especially those without proper health insurance, cannot afford to have a sonographic test performed.
It would be beneficial if there were an instrument a practitioner could use to measure the cervix quickly and accurately, and with little material expense. Although there are several instruments available for determining various dimensions of the uterus, there is no suitable instrument for measuring the length of the cervix in the formix vaginae. For example, U.S. Pat. No. 4,016,867 describes a uterine caliper and depth gauge for taking a variety of uterine measurements, which although useful for fitting an intrauterine contraceptive device, is not capable of measuring the length of the cervix in the formix vaginae due to interference by the caliper's wings. In fact, similar devices described in U.S. Pat. Nos. 4,224,951; 4,489,732; 4,685,474; and 5,658,295 suffer from similar problems due to their use of expandable wings or divergeable probe tips. These devices are also relatively sophisticated, making them expensive to manufacture and purchase. U.S. Pat. No. 3,630,190 describes a flexible intrauterine probe, which is particularly adapted to measuring the distance between the cervical os and the fundus of the uterus. The stem portion of the device has a plurality of annular ridges spaced apart from each other by a predetermined distance, preferably not more than one-half inch apart. However, this device is not adapted for accurately measuring the length of the cervix in the formix vaginae because of the lack of an appropriate measuring scale and a stop for automatically recording the measurement.
There exists a need for a simple and inexpensive device that can be used to determine the length of the cervix in the formix vaginae and, thus, predict the risk of preterm labor, as well as other conditions. There is also a need for such a device that can measure the dilation of the cervix uteri, to provide an overall assessment of the cervix and to determine the particular stage of labor. Ideally, the device should be adapted for use by a physician or obstetrician or even a trained nurse in the doctor's office or clinic. Preferably, the device should be sterile and disposable. In addition, it is desirable that device be able to lock after a measurement is taken to ensure that the measurement does not change between the time a user takes the measurement and removes the device from the patient to read the measurement. The present invention satisfies these needs and provides related advantages as well.